Method of melting refractory metals using a double heating process



1964 PA] YEN LOUNG METHOD OF MELTING REFRACTORY METALS USING A DOUBLE HEATING PROCESS Filed NOV. 15, 1962 'm m" nu mlm i mwmm INVENTOR Poi Yen Lou ng BY M, amid hm, M143 ATTORNEYS points.

arsaam METHQD 6F MELTHNQ REFRAQTQRY METALS USING A DtitlilLE HEATZNG PRKOCESS Pal Yen Losing, 311% David Ave, Palo Alto, (Calif. rune Nov. rs, 1962,ef. No. as7,as7 lll Qlaims. Cl. 75-49) This invention relates to a process and apparatus for melting metals and forming ingots therefrom and mpre particularly relates to a process and apparatus for melting 3,160,497. Patented Dec. 8, 1954 By following the teachings of the present invention it is possible to thoroughly and uniformly melt such refractory metals as tungsten, and to maintain the metal in molten form forextended periods of time; It is further" possible to produce rods or ingots directly from the molten metal, and if desired, to produce the rods on a and forming ingots from refractory metals," especially tungsten. V

Refractory metals, such as tungsten, molybdenum, tantalum, and niobium have extremely high melting For example, tungsten has the highest melting point of any pure metal, about 3,370" C.; tantalum has amelting point of about 2,850 C. Because of their high melting points, exceptional difliculties are encountered in melting these refractory metals and in molding or manufacturing articles from them.

Two major problems are involved in melting a refractory metal. The finst is the generation of sufiicient heat to melt the metal; the second is a means for containing the metal in its molten form.

In general, melting of metals such as tungsten is achieved by the arc melting or by the electron beam meth- In addition, the electron beam ing very high melting points and high vapor pressures this may interfere with the melting operation. The electron beam method is, moreover, an expensive method of melting.

Suitable means for containing the high melting point metals in their molten state are limited. The apparatus is either complex, presents contamination problems, or

both. For example, water-cooled copper containers as are commonly used in handling molten tungsten are likely to cause contamination of the metal, particularly at the surface of the tungsten ingot. An additional problem in usingsuch containers is that the temperature of the container must be carefully controlled to avoid extreme temperature differentials which might lead to explosion of the container. Where carbon containers are used, the tungsten will combine with the carbon to form tungsten carbide. It is apparent that the problems of containing the molten refractory metal also. apply to molds used in the molding of the metal.

Due in part to the difiiculties involved in melting, containing, and molding of the high melting point refractory metals, the art of powder metallurgy has developed. In the practice of powder metallurgy particles of the metal are pressed together under high pressure, and usually presintered, sintered, swaged, reheated, and swaged again. The steps involved in producing a rod or other articles are numerous, time consuming and costly. At no'point in the process is the metal in molten form because the highest sintering temperature is about 90-95% 0 1 the melting point of the metal. high the compression of the pO\Vd6I',Itl16 metal cannot be pressed into its crystalline form. Thus, manufacturing articles by powder metallurgy produces at best a relatively porous, Weak, brittle product which does not exhibit the true mechanical and physical properties of the metal.

Therefore, no matter how continuous basis. x p

The present invention is based upon the discovery "that a predetermined or localized portion of a sintered block of refractory metal, such as tungsten, may be melted to create a pool of molten metal in a crucible portion of the block, the crucible portion being formed as a result of the melting process itself. discovery that the crucible portion may be created by the concentration of heat generated by separate andcrossing or converging electric currents in a specific, predetermined localized area of the block without meltingother portions or" the block, and that the unmelted remainder of the block acts as an efiective insulator for the crucible comprising the steps of heating a' block of the refractory 1 metal and adding heat troma second source to a portion of the block to raise the temperature of that portion above the melting point. A molten pool of the refractory metal is thereby created. The ingot is then formed by contact- 'ing the molten metal poolwith an end of a rod made of the refractory metal and gradually withdrawing the rod from the pool to form an extension of the rod.

To illustrate the process and the apparatus of the in vention a preferred embodiment is described below with reference to the accompanying drawings. 1

FIG. l is an elevation of the principal parts of the furnace and shows a partial section of the block to illustrate the crucible portion.

FIG. 2 is a plan view of the apparatus shown in FIG. 1.

FIG. 3 is a-schematio illustrating an alternate arrangement of the block and the electrodes.

FIG. 4 is an elevation of another embodiment of the invention illustrating electrical induction means for heating the block.

Referring to FIGS. 1 and 2a sintered block of the re fractory metal'to be melted is designated generally as 1. I

The block 1 is supported by two insulating layers 2 and 3, which in turn are supported by a foundation 4. Attached to opposite ends of the block 1 are electricalconnecting means 5 and 5'. The electrical connectingmeans are preferably made of copperand have water-cooling Y jackets, not shown.

Disposed above the block 1 and in contact therewith are two spaced apart sintered electrodes 6 and 6'. The sintered electrodes 6 and 6' are spaced from each.

other and define edges of the limited portion of the surface ofthe block 1 which is to be melted. The electrodes 6 and 6 both contact the block 1 within the bounds of the block itself so that when the limited portion of connectors 7 and 7' are preferably made of copper and surrounded by a water-cooling jacket, not shown. Suit able support rods 8 ands hold the electrical connectors Y in place.

The partial section shown in FIG. 1 illustrates the crud It is" further based upon the ble portion 9 containing a pool of molten metal 10. The crucible 9 and the pool 10 are both formed as a result of the localized melting of the upper portion of the block 1, which process is to be described in detail later. Contacting the pool 10 is a sintered rod 11 made of the same.

refractory metal as that making up'the block 1 and the electrodes-6 and 6. Rod 11 acts as a seeding rod and is used as a means for. making an ingot or rod of metal, directly from the molten pool. 1

The drawings illustrate the principal features of'the invention. During operation the entire furnace should preferably be-under cover and inthe environment of an inert gas, hydrogen gas, or in some cases under a partial vacuum. Also not shown is means by which the seeding rod 11 maybe withdrawn, there'being a variety of means possible; nor is a means shown for adding pellets or particles of unmelted metal to the molten pool for operating the system. on a continuousbasis.

To form rods oringots of refractory metal the furnace isoperated by. first passing any electrical current through the block 1 via electrical connectors and 5. Because of the resistance of the metal comprising the block, the block becomes heated. It is noted that While all the refractory metals exhibit a generally high degree of resistivity, thefact that the block is sintered increases its resistance. This higher resistance of the sintered metal is due to its relatively porous nature, the particles not a being in asintimate relationship as in a crystalline molded.

product. By controlling the current, a sufficient amount can be passed through the block to cause it to heat to a temperature near the melting point of the metal comprising the block. As the resistance heating of theblock 1r progresses and the temperature approaches the melting point the electrical conductivity increases due to the closing of. the voids in the sintered material. This phenomenon isput touseful advantage in arranging thev electrical connectors 5 and 5' to direct current through a narrowed-part of the block 1, as best shown in FIG. 2. By

so directing the current, the side portions of theblock 12 and 13 remain at a. relatively lower temperature and therefore have-lower electrical and thermal conductivity. Thus,- the-relatively porous sintered block 1 acts as an insulator to contain and concentrate the heat generated by the electrical current. Similarly, concentration of the current in the upper portion of the block may be effected by other designs or arrangements of the electrical connectors 5 ands.

. Simultaneously with theheating of the block ll, or subsequent thereto, a second electric current is passed through the upper. centralportion of the block 1 by passing current from one electrode 6 through the block, and to the second electrode 6". Since the current will take the shortest distance available between the two electrodes 6 and 6' it will necessarily pass through the upper central portion. The fact that that portion is previously heated. will make it more conductive and more receptive fto current and further assist in localizing the current. The current passing through the electrodes 6 and 6' causes that portion of the block 1 located between the electrodes and somewhat below the surface of the block to be subiected to the energy from both electrical currents. Thus, that portion of the block 1 which is subjected to this cross-current heating becomes heatedto a much greater degree than the rest of the block.- By passing sutlicient additional current through the block 1 between the electrodes 6 and 6 it is possible to'raise the temperature of the block at that portion to a temperature above the melting point. When the melting point is reached that predetermined portion of the block 1 melts and a molten pool of metal is formed. At the same time, the melting of the metal causes a carving-out of the block at the area of crosscurrent heating thereby forming a crucible portion 9 in the block 1.

From the foregoing description it can be seen that electrical-heating from two distinct sources in two distinct form of electrical heating there described is resistance heating. Electrical heating of the block may also be, as in PH}. 4, inductance heating. In both these cases the heating is by reason of electric currents flowing in the block 11.

The crucible portion 9 is unique. It'is not permanent as is an ordinary crucible but is dependent upon several factors, particularly the input and output energy required for heating and the rates of charging and removing metal from the molten pool 10. By controlling these factors the size of the crucible'can be maintained Within of the furnace. 7

After the molten pool of metal 10 has been created in the cruciblepor'tion 9 of the block 1, the rod or ingot 15 formed directly from the molten pool," as best illustrated in FIGURE 1. There a sinter'ed rod 11 made of the same refractory metal as comprises the block 1 and electrodes 6 and 6 is dippedinto the pool 10. Upon contact with-the pool 10, the molten metal immediately surrounding the end of the rod 11 will cool due to the lower temperature ofthe rod and thus crystallize upon the end of the rod. As the rod 11 is slowly Withdrawn from the molten pool 10, metal will continue to crystallize upon the built-up and successively cooled portion at the rods end... In this manner an extension of the rod 11 is continuously formed. The extension so formed is metal in its pure crystalline state. and exhibits all the characteristics'of the. true metal. It is less porous, less brittle, and more ductile than a similar rod formed by i sintering.

' dispersion of heat from the molten pool.

By practicing this method. of ingot formation is can be seenthat the method is particularly adaptable to continuous operation. Continuous operaton can be performed by simply charging the molten pool with pellets. or other particles of the metal at a rate commensurate Wltll the withdrawal of metal from the pool.

In contrast, if batch production is desired the electrodes 6 and 6' may be used to charge the pool, by gradually dipping them into the pool, thus consuming the electrodes.

A number of advantages are gained by following the teachings of this invention.

The melting 50f a'portion of the sintered block, therekeep the container from melting. Because of the porous,

nature of the sintered block with its .multitudinous voids, the block also acts as an insulator to prevent the rapid Since it is possible to maintain a pool of the metal for extended periodsiof time, impurities in the metal such as iron, nickel, cobalt, manganese and chromium are given an opportunity to volatilize and escape, thereby further purifying the metal to an extremely high degree. Maintenance of a molten-pool also provides thorough and uniform melting.

The concept of using a seedingrod upon which to build an ingot has the advantage of avoiding intervening steps such as pouring and molding to form an ingot. The advance over the numerous steps involved in powder metalliir'gy is apparent. v V

The principal advantage, however, is that rods or ingots so formed exhibit the characteristics of the true.

metal because natural crystallization is permitted. Thus, they are in better conditionfor further. processing such as wire-drawing. Further, by controlling the size of the seeding rod, and the rates of cooling and withdrawal,

relatively long single crystals will form, thus enhancing the ductility of the metal. Crystallization further purifies the metal.

To illustrate the size and power requirements involved in the practice of the invention the following example is given. A block of sintered tungsten having length of 12 and width of 5" and a depth of 1 is used as the main component of the furnace. Electrical connectors, suitably water-cooled, having a width of 3 are afiixed to either end of the block. Two electrodes of sintered tungsten, each having a length of 3", a width of 3" and a thickness of A" are disposed'above the block and each has one end in contact with the top surface of the block. The electrodes are spaced apart and opposite to each other. Aflix'ed at the other end are water-cooled electrical connectors. In operation the furnace is placed under the cover of an inert gas and the watercooling circuluation started. Electrical current of approximately 10,000 amps is. passed through the main block, heating the middle ,portion to about 3,000 C. A second electrical current 'of about 5,000 amps is passed through the electrodes and through the upper central heated portion of the block, heating that portion to over 4,000" C. and melting it.

One end of a sintered tungsten seeding rod is then introduced contacting themelted part, which is a pool in' a crucible portion of the block. The seeding rod is then slowly withdrawn, and due to the cooling as the rod is withdrawn, the tungsten crystallizes on the end forming an extension.

A number of embodiments of the invention may be made without departing from the spirit of the invention.

For example, as shown in FIGURE 4 alternate means of heating the main block 1 are available. In that figure electrical induction means 14 are used in combination with resistance heating by the electrodes 6 and 6'. It would also be possible to have various combinations of resistance heating, induction heating, solar heating, are heating, electron beam heating and plasma heating without departing from the concept of heating a main block and concentrating heat generated from a second source at a portion of the block to cause localized melting of the block.

The arrangement of the electrodes is not limited to the embodiments shown. For example, several pairs of electrodes may be used and the paths taken by the electrical currents may be directed differently. The result to be achieved is the crossing of the currents at a given predetermined portion of the block to cause concentration of the heat.

In the present embodiment, upward withdrawal of the metal by means of a seeding rod is contemplated. However, the metal could be removed from below or by'other conventional means. Where a seeding rod is used, there is an additional advantage in that the rod couldbe run directly through rollers or a die thus saving steps in the continuous process.

I claim: p

1. The method of melting and maintaining a refractory metal in molten form, said method comprising (a) electrically heating a sintered block of said refractory metal, and

(Z2) electrically heating a limited portion within the surface boundaries of said block to a temperature above melting point by passing an additional electriccurrent through said limited portion, thereby creating a molten pool of said metal within the surface boundaries of said block.

2. The method of melting and maintaining a refractory metal in molten form, said method comprising (a) electrically heating a sintered block of said refractory metal by passing a first electric current through said sintered block, and

(b) electrically heating a limited portion Within the upper surface boundaries of said block to a temperature above the melting point by passing a second electric current through said limited portion, thereby creating. a molten pool of said metal within said.

' limited portion."

3. The method of melting and maintaining a refrac- 5 tory metal in molten form, said method comprising (a) electrically heating a sintered block of said re- 4. The method of melting and maintaining tungsten in molten form, said method comprising sten, and (b) electrically heating a limited portion with the surface boundaries of said block to a temperature above the melting point by passing an additionalclectric current through said limited portion, thereby creating a molten pool of said metal within the surface boundaries of said block. 1

p 5. The method of melting and maintaining tungsten'in,

molten form, said method comprising tered block, and

(b) electrically iheatinga limited portion within the upper surface, boundaries of said block to a temperature above'the melting point by passing a second electric current through said limited portion,

thereby creating a molten pool of tungsten within said limited portion.

6. The method of melting and maintaining tungsten in molten form, said method comprising (a) electrically heating a sintered block of tungsten.

by inducing an electric current in said sintered block, 40 and (b) electrically heating a limited portion within the" surface boundaries of said block to a temperature above the melting pointby passing a second electric inga molten pool of said metal Within said limited portion. 7. The method of forming an ingot of a refractory metal, said method comprising tory metal,

(b) additionally electrically heating a limited portion within the bounds of said block to raise the temperature of said limited portion above the melting point, thereby creating a molten pool of said metal within said limited portion,

(0) contacting said molten pool with one end of a rod made of said refractory metal, and i (d) gradually Withdrawing said rod from said pool to form an extension of said rod.

tric current through said limited portion, thereby 7 creating a molten pool of said metal within said limited portion, V (c) contacting said pool with one end of a rod made of said refractory metal, and (d) gradually withdrawing said rod from said pool, to form an extension of said rod.

(a) electrically heating asintered block of said tungcurrent through said limited portion, thereby creat- (a) electrically heating a sintered block of saidrefrac- 8. The method of forming an ingotof a refractory 7 9. The method of forming an ingot of a' refractory metal, said method comprising a V I (a) electrically heating a sinteredblock of said refractory metal by inducing an electric current in said sintered block, (b) electrically heating a limited portion within the upper surface boundaries of said block to a temperature above the melting point by passing a second electric current through said limited portion, thereby creating a molten pool of said metal within said limited portion, 3 7' V j (c) contacting said pool Withone end of a rod made of said refractory metal, and r p r (d) gradually withdrawing said rod" from said pool to form an extension of said rod. 10. The method of continuously forming an ingot ofrefractory metal, saidmethod comprising 7 (a) electrically heating a sintered block of said refractory metal to a temperature near the melting point of said block,

(b) electrically heating a limited portion within the 1 boundaries of the upper surface of said block to a temperature above the melting point by passing a second electric current through said limitedportion, thereby creating a molten pool of said metal Within said limited portion, l

1,580,199 Hering Apr. 13', 1926.

- 2,159,169 Mautsch; 23 ,119.39"

2,782,1'14 Preston Feb. 19, 1957 2,809,136 Mortimer Oct. 8, 1957' 2,845,261 Furczyk -L July 29, 1958 2,858,199 Larson, Get; 28?, 1958? 2,878,011 Sherban Mar. 17, 1959 2,890,109 Cooper 'Iune 9, 1959:

12,904,430 Taylor et al. Sept. 15, 1959 2,962,363 Martin .L .,Nov. 29, 1960' 2,977,258 Dunkle Mar. 28, 1961 2,979,386 Shockley et all Apr. 11, 1961 2,997,385 Winter Aug. 22, 1961 3,020,128 Adcock et a1 Feb. 6, 1962 Takeuchi June 19, 1962 8 (c) contacting said pool with one end of rod made 7 of said refractory metal, a (d) gradually withdrawing said rod from said pool to form an extension of said rod, and ('e) continuously adding unmelted particles of said refractory melted to said pool. 2

References Cited in the file of this patent VUNITED STATES PATENTS Paient No, 3, 160,497 December 8, 1964 Pai Yen Loung It is hereby certified that error appears in t hat the said Letters ent reqiiring correction and t corrected below.

he above numbered pat- Patent should read as Column 6, line 7, for "including' read inducing line 19, for "with" read within Signed and sealed this 4th day of May 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Aitcsting Officer Commissioner of Patents 

7. THE METHOD OF FORMING AN INGOT OF A REFRACTORY METAL, SAID METHOD COMPRISING (A) ELECTRICALLY HEATING A SINTERED BLOCK OF SAID REFRACTORY METAL, (B) ADDITIONALLY ELECTRICALLY HEATING A LIMITED PORTION WITHIN THE BOUNDS OF SAID BLOCK TO RAISE THE TEMPERATURE OF SAID LIMITED PORTION ABOVE THE MELTING POINT, THEREBY CREATING A MOLTEN POOL OF SAID METAL WITHIN SAID LIMITED PORTION, (C) CONTACTING SAID MOLTEN POOL WITH ONE END OF A ROD MADE OF SAID REFRACTORY METAL, AND (D) GRADUALLY WITHDRAWING SAID ROD FROM SAID POOL TO FORM AN EXTENSION OF SAID ROD. 